JPH03199308A - Production of steel material having excellent drilling property - Google Patents

Abstract

PURPOSE:To produce the steel material having an excellent drilling property by forging and hot rolling a steel ingot near the crater end thereof in such a manner that the ratio of the content of C in the axial central part of the ingot with respect to the content of C in the molten steel in a ladle attains a specific value at the time of continuous casting of the molten steel. CONSTITUTION:The molten steel having the compsn. contg., by weight%, 0.2 to 1.0% C, 0.1 to 1.5% Si, and 0.3 to 2.0% Mn, is forged near the crater end where the unsolidified molten steel in the ingot completes solidification in such a manner that the value of the ratio C/C0 of the carbon content C in the axial central part of the ingot with respect to the carbon content C0 of the molten steel in the ladle attains 0.70 to 1.05 at the time of continuously casting the above-mentioned molten steel in the ladle and producing the steel material suitable for steel parts requiring drilling. In succession, the bloom in produced by hot rolling, then after the bloom is hot rolled to a billet by blooming and ingot mill, the billet is hot rolled to large-diameters bars, steel bars, bar-in coils and wire rods. The steel product having the excellent drilling property are thus produced.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention is applicable to shafts and pins having nuts, collars, and lubricant supply holes, or various mechanical parts that require internal processing using a lathe or drill. The present invention relates to a method for producing steel materials that are advantageously suited for products that require processing.

(Prior art) In general, the above-mentioned products are processed by finishing the rolled material into the desired dimensions and shape, drilling holes, and then using quenching, carburizing, nitriding, etc., depending on the use of the product. The surface is hardened.

The steel materials used for the above processing have excellent workability such as cutting, cutting, and grinding when finishing rolled materials into desired dimensions and shapes.
Furthermore, the subsequent drilling process requires excellent workability.

Therefore, it is important for steel materials to be free of internal defects.In particular, center segregation, which occurs in the center of steel materials, is a problem that occurs when machining a hole that passes through the center of a rolled material or a hole that extends in the axial direction of a bar material. This may impede productivity and lead to a decrease in productivity and yield.

In the case of slabs obtained by continuous casting, the center segregation that causes such problems is caused not only by solidification shrinkage at the solidified tip, but also by the vacuum absorption force of the voids caused by bulging of the solidified shell, resulting in the center segregation at the solidified tip. As a result of the suction of concentrated molten steel components such as As a result of precipitation of cementite and non-uniformity of the microstructure, the machinability of hole drilling, typified by the straightness of hole drilling, deteriorates.

As a measure to prevent such center segregation, attempts have been made to, for example, use electromagnetic stirring in the secondary cooling zone, but this has not been able to reduce even semi-micro segregation, and the effect cannot be said to be sufficient.

In addition, the so-called in-line reduction method (Tetsu-to-Hagane No. 60
(1974) No. 7, pp. 875-884), but with this method, if the reduction in the area of the slab with a large unsolidified layer was insufficient, cracks would occur at the solidification interface, and the opposite effect would occur. If the rolling reduction is sufficient, there are problems such as strong negative segregation occurring at the center of the thickness of the slab.

In addition, JP-A No. 49-121738 discloses a method in which a light reduction is applied with a pair of rolls near the solidified tip of a slab to compensate for the amount of solidification shrinkage in this area by reduction. Japanese Patent No. 54623 proposes a method of greatly reducing the area near the solidification completion point of a slab using a casting mold.

However, in the case of light rolling by rolls, even if several pairs of rolls are used to roll down by several degrees, it is not possible to sufficiently prevent solidification shrinkage and bulging that occur between the roll pitches, and the rolling position is If it was not appropriate, there would be a disadvantage that center segregation would worsen. On the other hand, the method of applying a large reduction near the solidification point of the slab using a forging die makes the solidification interface less likely to crack than the in-line reduction method, which uses large reductions using rolls, and also avoids negative segregation as much as possible. However, if the reduction in the area of the slab with a large unsolidified layer is insufficient, cracks will occur at the solidification interface, and conversely, if the reduction is too large, cracks will occur in the slab. There is a disadvantage that strong negative segregation occurs in the center, and furthermore, the effect cannot be obtained even if a small area of the unsolidified layer is rolled down, so the current situation is to find the optimal rolling conditions.

Therefore, it has not yet been possible to dramatically improve the center segregation that occurs in slabs, and the reality is that depending on the steel type and application, diffusion annealing is applied at the slab stage to deal with it.
A significant increase in cost is inevitable, and in particular, drilling holes in the center of the rolled material poses a problem as it reduces the straightness during processing.

(Problems to be Solved by the Invention) This invention advantageously solves the above problems, and even when using a continuous casting method, the generation of center segregation is reduced as much as possible, so that hole drilling is reduced by machining. The purpose of this study is to propose an advantageous manufacturing method for steel materials with excellent workability.

(Means for Solving the Problems) That is, the present invention provides C: 0.2 to 1.0 wt% (hereinafter simply expressed as %), Si
: 0.1 to 1.5% and Mn: 0.3 to 2.0%, with the remainder consisting of Fe and unavoidable impurities. At that time, the molten steel inside the slab solidifies. Near the completed crater end, the C content of the molten steel in the ladle (
The cast slab is subjected to forging processing such that the ratio C/CO of the C content (C) at the axial center of the slab to C0) is 0.70 to 1.05, and then hot rolled to form thick bars, steel bars, bar-in coils and Drilling, which is characterized by the use of wire rods, is a method for manufacturing steel materials with excellent workability.

(Function) First, the reason why the composition of molten steel is limited to the above range in this invention will be explained.

C: 0.2-1.0% Since it is mainly used for mechanical parts, it is necessary to guarantee the strength after processing, and the lower limit of the C content is set at 0.2%.

On the other hand, the higher the C content, the higher the strength of the steel wire, but on the other hand, increasing the C content makes the material brittle and increases the frequency of wire breakage during processing. In particular, if it exceeds 1.0%, mesh-like cementite will precipitate at the prior austenite grain boundaries during controlled cooling after wire rolling, which will greatly impede subsequent wire drawability, so the upper limit should be 1.0%. Established.

Si: 0.1-1.5% Si is an element useful not only as a deoxidizing agent but also for strengthening the matrix, and requires at least 0.1%. On the other hand, Si has the effect of increasing the activity of C,
In particular, if the content exceeds 1.5%, the formation of a decarburized layer becomes noticeable, resulting in insufficient surface hardness, requiring a step to remove the decarburized layer, and also resulting in a decrease in yield. For this reason, the Si content was set in the range of 0.1-1.5%.

Mn: 0.3-2.0% Like Si, Mn not only acts as a deoxidizing agent, but also fixes S, which causes embrittlement of steel, and also improves hardenability and increases strength. Although it is a useful element for increasing ductility, if the content is less than 0.3%, its addition effect will be poor, while if it exceeds 2.0%, it will not only be expensive but also require controlled cooling after hot rolling or Since it promotes the formation of the chromartensite structure in the heat treatment step during processing and the formation of holes impairs workability, it was added in a range of 0.3 to 2.0%.

In the present invention, during continuous casting of molten steel having a suitable composition as described above, the C content of the molten steel in the ladle is reduced by performing forging near the crater end where the molten steel inside the slab completes solidification. The ratio C/C of the C content (C) in the axial center of the slab to (Co) is 0.70 to 1.
05.

The reason why the C/CO ratio can be controlled by forging is as follows.

In other words, at the final stage of solidification of the internal molten steel, molten steel with advanced C concentration exists near the crater end, so if it solidifies as it is, it will become centrally segregated, but if forging is performed before solidification, this will occur. As a result of the C-enriched molten steel being pushed upward, the C concentration in the center does not increase much. Therefore, by adjusting the timing of forging according to the degree of enrichment of C, the C content in the axial center of the slab can be adjusted.

Next, the continuously cast slabs subjected to continuous forging according to the present invention and those obtained according to the conventional method without forging are rolled into steel bars, and the axial center of the slabs is drilled. When we investigated the drill life during machining, we found that
The lifespan of drills that were subjected to forging was 1.2 to 1.5 times longer than those that were not subjected to forging.

Furthermore, the C
Steel materials with various C/Co ratios were sampled, rolled into steel bars, and then the shaft center was drilled with a drill.The straightness of the drill was investigated, and it was found that the C/Co ratio was 0.
In the case of a large negative segregation rate of less than 70, and conversely, C/c,
In any case where the ratio is greater than 1.05 and the positive segregation rate is large, the straightness of the drill decreases. The reason for this is thought to be that when the negative segregation rate becomes large, it induces the swing of the drill, and when the positive segregation rate is large, the drill escapes to the negative segregation side at the boundary of the part where the difference in component concentration occurs. It will be done.

Therefore, in this invention, the C/CO ratio in the axial center of the slab to be controlled by forging is limited to a range of 0.70 to 1.05.

In addition, as a preferable forging method, there is a continuous forging method previously disclosed by the inventors in JP-A-60-82257.

In addition, by the forging method according to the above-mentioned method of the invention, C/
C. The slabs with a controlled ratio of 0.70 to 1.05 are made into steel bars by hot rolling, and among these bars, the highest hardness in the center 78.5 mm in the cross section and the average hardness of the matrix When drilling holes with various hardness ratios, it was found that the drill life and straightness decreased rapidly when the hardness ratio exceeded 1.1.

(Example) Molten steel (symbols A-H) with the chemical composition shown in Table 1 was
C/C of the axial center of the slab in the vicinity of the crater end where the molten steel inside the slab completes solidification for the slab that is continuously cast in a 0x340mm mold and is being drawn.

ratio? Continuously press-processed with a target of 0.80,
A plume was manufactured by controlling the C/CO ratio within a range of 0.70 to 1.05. Then 1 by blooming and billet mill
Hot rolled into a 50 x 150 mm billet. Furthermore, it was hot rolled into a 40 mmφ steel bar using a steel bar mill.

On the other hand, the comparative material was subjected to the conventional process of continuous casting and then to bar rolling without forging.

The molten steel was heated at a temperature of 25 to 30° C. during tapping. In addition, consideration was given to the hot rolling temperature from blooming to bar rolling so that both the steel material obtained according to the present invention and the comparison material had the same temperature history.

Table 2 shows the results of investigating the mechanical properties of these rolled steel bars.

Here, the tensile test specimens are shown as the average value of a total of 30 specimens continuously sampled from the rolled material at both ends and the middle of the billet. Regarding other properties, the results of an investigation of samples taken from the immediate vicinity of the tensile test piece are shown. Note that the center spot was expressed as presence or absence regardless of its degree.

( wt

In contrast, center spots were observed in all comparative materials.

Such a difference appears in the aperture, and the conforming example clearly shows a higher value for any steel material.

Next, in the obtained steel bar, the life of the drill was investigated when drilling a hole toward the center axis of the bar was performed. The results are shown in Fig. 1.

As shown in the figure, when the drill life of the steel type A that was not subjected to forging processing was set as 100, the drill that was subjected to forging processing had a long life and was advantageous for all steel types.

In addition, as shown in Figure 2, the results of investigating the straightness of the drill when the above-mentioned drill hole was processed are shown as a relationship with C/Co. 0.8
It was highest in the range of 5, while it rapidly decreased when it exceeded 1.10.

(Effects of the Invention) According to the present invention, by continuously applying forging during continuous casting and controlling C/CO at the axial center of the slab, holes in the steel material after hot rolling can be made with straightness during processing. Drilling can provide steel materials with improved workability, and at the same time, drilling can extend the life of the drill used for processing.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between rolled steel bars and drill life for each steel type, and Figure 2 is a graph showing the relationship between C/C ratio of rolled steel bars and drill straightness. @1 Figure a trend ClCO ratio

Claims (1)

[Claims] 1. Contains C: 0.2 to 1.0 wt%, Si: 0.1 to 1.5 wt%, and Mn: 0.3 to 2.0 wt%, the remainder being Fe and inevitable impurities. Molten steel with a composition of
The ratio C/C_0 of the C content (C) in the axial center of the slab to C_0) is 0.70 to 1.05.Then, hot rolling is performed to form thick bars, steel bars, bar-in coils and A method for manufacturing a steel material with excellent hole-drilling properties, characterized by making it into a wire rod.